brookite

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A mineral form of titanium dioxide, TiO₂, having characteristic orthorhombic crystals and a red-brown to black color.

Century Dictionary and Cyclopedia

1. n. One of the three forms in which titanium dioxid occurs in nature. It is found in orthorhombic crystals of a brown or yellow color to black, and adamantine to metallic luster. Jurinite is another name for the same mineral. Arkansite is an iron-black variety from Magnet Cove, Arkansas.

Wiktionary

1. n. A dark brown mineral form of titanium dioxide, with the chemical formula TiO2.

GNU Webster's 1913

1. n. A mineral consisting of titanic oxide, and hence identical with rutile and octahedrite in composition, but crystallizing in the orthorhombic system.